The present invention relates to methods of fabricating optical waveguides for selectively filtering light according to wavelength as it propagates within the waveguide.
Optical waveguides comprise a core region having a first refractive index and surrounding cladding regions having a second, lower refractive index. In an optical fiber waveguide the core region has a circular cross section and is surrounded by a substantially thicker cladding layer. The interface between the core and the cladding is substantially planar such that transmission occurs primarily in the core-guided fundamental HE11 (zero order) mode, thus preventing energy loss through the cladding layer. In many applications optical waveguides convey polychromatic light. Fiber Bragg Gratings (FBG) permit the selective filtering of the light according to wavelength within such optical waveguides, and among other benefits, avoid interrupting the waveguide to insert macroscopic filters. Generally speaking, Fiber Bragg gratings are periodic structures formed in optical fiber or other optical waveguides. The exact nature of the wavelength selective filtering is determined, among other factors, by length and repetitive sequence of the periodic structures. In one type of Fiber Bragg grating, known as a Bragg Reflection Grating, discrete regions of different refractive index define the periodic structure that traverse the core of the optical fiber. The length and order of these discrete regions need not repeat exactly to form a periodic structure, as the structure is selected to achieve a particular transmission value that may vary over a predetermined wavelength range. The periodic fluctuations in refractive index typically modify the transmission characteristics of the waveguide by constructive and destructive interference on the basis of wavelength, such that selected wavelengths are partially or totally transmitted or reflected. However, the FBG structure may also modify the phase, polarization or other propagation characteristics of the incident light.
The wavelengths not transmitted in a Bragg Reflective Grating are reflected by the periodic structure such that they propagate in the waveguide core in the reverse direction. The reflective grating is typically fabricated by xe2x80x9cwritingxe2x80x9d the desired the grating structure in an optical glass whose refractive index is dependent upon the dose of ionizing radiation absorbed therein. The xe2x80x9cwritingxe2x80x9d process being the elected exposure of the periodic regions within the optical fiber to ionizing radiation. These writing methods include a flood exposure with UV light through a patterned mask, as well as direct writing with a focused light source or other form of ionizing radiation. The resultant grating can reflect light of wavelength xcexc provided the Bragg condition, xcexc=2nxcex9, is satisfied in which n is the mode refractive index and xcex9 is the grating period or pitch. Accordingly the pitch of the index fluctuation along the length of the fiber is comparable to the wavelength of light to be filtered by reflection.
A second type of Fiber Bragg Grating filters selected wavelengths by coupling to a forward propagation mode in the cladding, rather than by back reflection in the core, as in a Bragg Reflection Grating. The periodic structure for mode coupling between the fiber core and cladding can be introduced by either reducing the refractive index difference between the core and cladding in selected regions, as taught in U.S. Pat. No. 5,764,829, supra, or by periodically distorting the planar interface between the core and cladding layers. The term Long Period Grating is descriptive of the longer structural periodicity relative to a Bragg Reflection Grating having the same center wavelength position. The transmission profile of a Long Period Grating is the square of the sinc function (sin x/x), having a center wavelength position according to the relationship:
xcexc=(ncxe2x88x92ncl)xcex9,
wherein nc is the core effective index and ncl is the nth cladding mode effective index. As this difference in index is a small number, that is usually less than {fraction (1/10)}th of nc the pitch to achieve the same center wavelength, xcexc can be an order of magnitude larger than in a Bragg Reflection Grating.
Long Period Gratings can be fabricated by selective UV exposure, for example as disclosed in U.S. Pat. No. 5,764,829, which is incorporated herein by reference. However, as the size of the periodicity is larger than for a functionally equivalent Bragg Reflection Grating they are advantageously fabricated by other methods. For example, it is disclosed in GB 2 155 621 that, by pressing an optical fiber against a ribbed surface so as to induce microbending with a particular periodicity, mode coupling can be induced at a selected wavelength between a mode guided by the core of the fiber (core mode) and one or more modes which are guided by the cladding (cladding modes), and which are attenuated relatively highly in comparison with the core mode. Such a device operates in transmission to attenuate selectively light at the wavelength at which such mode coupling occurs.
A letter by C. D. Poole et al. entitled xe2x80x9cTwo-mode spatial-mode converter using periodic core deformationxe2x80x9d, Electronics Letters Aug. 15, 1994 Vol. 30 No. 17 pp 1437-8, discloses how a mode coupling filtering effect may be achieved by an ablation method. This method creates period changes in curvature by first using a laser to ablate material from the side of the fiber at equally spaced localized intervals along its length. Subsequent heating of those regions of the optical fiber produces a similarly localized wrinkle in the core of the optical fiber due to surface-tension effects. The authors of this letter suggest that an advantage of the ablation method over the photo-induced grating method is that the perturbations achievable by photo-induced refraction index changes are limited in magnitude to inconveniently small values, whereas the ablation method can be used to write much larger perturbations.
U.S. Pat. No. 5,708,740, which is incorporated herein by reference, is similarly directed to a method of producing mode-coupling optical fiber notch filters whose periodic perturbations are comparable with those readily achievable using the ablation method, but which is distinguished from the ablation method, inter alia, in that each perturbation is created using a single stage process, such that the creation of the perturbations does not involve any deliberate off-axis deviation of the fiber core.
Fabrication errors present a practical limitation in the commercial manufacture of Long Period Gratings, in that all techniques of fabricating Fiber Bragg Gratings that function by mode coupling variations are subject to tolerance errors and deviations from multiple causes. The errors cannot generally be tested without completing the grating fabrication and measuring the actual transmission function of the grating by spectroscopic methods. As some of the errors are random, the yield of suitable product is lowered.
It is therefore a first object of the present invention to provide a method for reworking fabricated Long Period Gratings so as to correct for fabrication errors.
Another objective is to provide a method for shifting the filter characteristics of Long Period Grating without further mechanical deformation or other changes in physical dimensions.
Yet another objective is to provide a method for thermally stabilizing Long Period Grating that have modified or shifted filter characteristics obtained without mechanical deformation or other changes in physical dimensions.
In the present invention, the first object is achieved by fabricating a Long Period Grating by conventional methods using an optical waveguide having a core region that is photosensitive. After the Long Period Grating is formed at least a portion of the core region is exposed to ionizing radiation to modify the refractive index of the core region, and shift the profile of the wavelength dependent transmission. In the case of a photosensitive core optical medium that increases in refractive index upon UV exposure the shift of the transmission profile is to longer wavelengths.
The second objective of providing a thermally stabilizing Long Period Grating is achieved by annealing an irradiated Long Period Grating for a sufficient time, at a temperature dependent on the ultimate use temperature. Although annealing also shifts the wavelength dependence of transmission in the reverse direction as UV exposure, the steps of irradiation and annealing may be carried out in multiple sequences to adjust the filter performance and optimize the thermal stability of the filter over the anticipated operating temperature range. Alternatively, by selecting the irradiation dose to provide a greater than desired shift this reversal by annealing can be accommodated. Additionally, cycles of repetitive irradiation and annealing provide a method for post fabrication modification to meet different or changing end user requirements, as well as for tuning the filter to meet the initial targeted specification.
The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.